A brushless direct current (BLDC) motor has a magnet rotor and a stator with at least one winding wound thereon. Power can be supplied to the motor by an AC power supply via a power converting circuit. FIG. 1 illustrates signals of voltage, current and power of an AC power supply, based on a traditional power converting circuit. At the instant the motor is connected with the AC power supply, current passes through the winding of the stator of the BLDC motor and energy is stored in the magnetic field generated by the winding. As the winding is an inductive element, the phase of the current signal is lagging compared to the phase of the voltage signal. At the instant the motor is connected with the power supply, the rotor of the motor is stationary and no back electromotive force (EMF) is generated by the winding. With an increase in the voltage, the electrified winding increases the strength of a generated magnetic field which interacts with the magnetic field of the rotor to drive the rotor and the winding therefore generates a back EMF VB. According to the traditional power converting circuit, the winding is charged during the entire period of time the AC voltage is greater than the back EMF VB and the energy stored by the winding is relative high. After the AC voltage decreases to a value smaller than the back EMF VB, the power supplied from the AC power is stopped and the winding starts to discharge energy, which generally lasts until a moment after the AC voltage passes zero, as shown by the shadowed part in FIG. 1, and negative power is therefore generated. The negative power causes part of the power to flow back to the power network and consumes electrical energy, thereby resulting in a decrease in the effective power of the AC power supply.